An artificial heart is known in the art which delivers the blood to the aorta by withdrawing it from the heart as an example of an assisting in the functioning of a heart. However, it is undesirable to apply a significantly strong external action upon the heart by applying a negative pressure to withdraw the blood which exceeds the capability of the heart to feed the blood. For this reason, it is desirable to provide a pumping action which only assists in the discharge from the heart, in effect, by delivering the blood fed from the heart under pressure while maintaining the flow rate.
Japanese Laid-Open Patent Application No. 94,171/1987 discloses a pump unit including a pair of pumps which are connected in parallel to each other between the left atrium of the heart and the aorta so that during a period when one of the pumps contains a blood discharged from the heart (diastole), the other pump is pressurized to deliver the blood which is contained therein under pressure to the aorta (systole), the both pumps being operated in alternate fashion. Each pump includes an operating pressure chamber to which a drive pressure is applied and a sack disposed within the operating pressure chamber to surround a space into which a blood is received. In order to prevent an excessive negative pressure from being applied to the heart, the operating pressure chamber of the pump is made to communicate with the atmosphere during the diastole so as to allow the blood to enter the blood receiving space within the sack of the pump autonomously under the influence of the discharge pressure of the heart. The sack is responsive to the pressure within the operating pressure chamber to contract, shrinking the blood receiving space whenever the pressure is above the atmosphere, and to expand, increasing the blood receiving space in response to the pressure of the blood which flows into the blood receiving space whenever the operating pressure chamber communicates with the atmosphere. In order to monitor the contracting/expanding motion of the sack, a location on the sack which undergoes a reciprocating movement of a maximum stroke is detected, and the value of a positive pressure applied to the both pumps is controlled in a manner dependent on a time interval counted from the arrival of the given location on the sack of one pump to a given expanded position until a corresponding location on the sack of the other pump reaches a given contracted position, such that such time interval is substantially zero, meaning that the termination of the expansion (suction) of the one pump occurs concurrently with the termination of the contraction (discharge) of the other pump, whereupon the diastole and the systole of the both pumps are changed. In this manner, the pump unit is effective to deliver the blood under pressure to the aorta without causing a disturbance in the flow rate which is discharged from the heart. Since the pumped or delivered flow rate (or drive flow rate) varies automatically tracking a variation in the discharge flow rate from the heart, the pumping action cannot cause any significant loading on the heat. In this manner, an assistance in the functioning of the heart which deliver the blood is realized in a manner which suitably fits the actual operating condition of the heart or the physiological status of a living body, in particular, a change therein.
It will be appreciated that when the living body or the heart recovers, the pump unit must be removed from the living body. However, it is difficult to determine the degree of recovery of the living body or the heart while the pump unit is being used to assist in the blood feeding action of the heart.
Such determination will be greatly facilitated if the operation of the pump unit is stopped. However, if the operation of the pump unit is stopped when the assisting in the blood feeding operation is actually needed, there results a significant risk upon the living body. Accordingly, the need for an assistance by the pump unit must be determined by seeing if the actual flow rate being delivered does not exhibit a substantial decrease or if the actual flow rate being delivered decreases in a manner corresponding to a decrease in the assisting functioning of the pump unit or by seeing a response of the living body while gradually decreasing the blood feeding assisting function (drive flow rate) of the pump unit. Specifically, if the blood is fed under the influence of the discharge pressure from the heart to pass through the pump unit to appear at its outlet to show no change in the actual flow rate being delivered or if a reduction in the flow rate being delivered is small enough to deny an indication of a physiologically bad condition of the living body when the flow rate being delivered by the pump unit (or the drive flow rate) is reduced, a satisfactory recovery of the living body or the heart can be declared. However, it is difficult with the described pump unit to perform such adjustment, namely, reducing the assisting effect upon the blood feeding action in a gentle and smooth manner.
With an ordinary pump, the flow rate being delivered (or the drive flow rate) can be regulated by reducing a drive pressure. However, in the pump unit as described above, when the drive pressure is changed, the drive pressure will be automatically regulated so that the flow rate being delivered substantially matches the flow rate of the in-flow, and accordingly a change in the drive pressure merely results in a temporary disturbance in the pumping action without permitting an adjustment of the contribution of the pump unit in controlling the flow rate being delivered.